Multi-channel non-invasive tissue oximeter

ABSTRACT

A method and apparatus for spectrophotometric in vivo monitoring of blood metabolites such as hemoglobin oxygen concentration at a plurality of different areas or regions on the same organ or test site on an ongoing basis, by applying a plurality of spectrophotometric sensors to a test subject at each of a corresponding plurality of testing sites and coupling each such sensor to a control and processing station, operating each of said sensors to spectrophotometrically irradiate a particular region within the test subject; detecting and receiving the light energy resulting from said spectrophotometric irradiation for each such region and conveying corresponding signals to said control and processing station, analyzing said conveyed signals to determine preselected blood metabolite data, and visually displaying the data so determined for each of a plurality of said areas or regions in a comparative manner.

This application is a national stage of International Application No.PCT/US99/22940, filed Oct. 13, 1999, which claims the benefit of U.S.Provisional Application Ser. No. 60/103,985, filed Oct. 13, 1998.

Notice: More than one reissue application has been filed for the reissueof U.S. Pat. No. 6,615,065. Reissue application Ser. No. 11/219,298 waspreviously filed for the reissue of U.S. Pat. No. 6,615,065. The presentapplication is a Continuation Reissue Application of Reissue applicationSer. No. 11/219,298. Three other Continuation Reissue Applications ofReissue application Ser. No. 11/219,298 are filed on the same day asthis Continuation Reissue Application, specifically, ContinuationReissue application Ser. No. 13/780,300, Continuation Reissueapplication Ser. No. 13/780,314, and Continuation Reissue applicationSer. No. 13/780,326, all having the same title and same inventors asU.S. Pat. No. 6,615,065.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation Reissue Application of Reissueapplication Ser. No. 11/219,298, which was filed for the reissue of U.S.Pat. No. 6,615,065, which is a national stage of InternationalApplication No. PCT/US99/22940, filed Oct. 13, 1999, which claims thebenefit of U.S. Provisional Application Ser. No. 60/103,985, filed Oct.13, 1998.

This invention relates generally to in vivo spectrophotometricexamination and monitoring of selected blood metabolites or constituentsin human and/or other living subjects, e.g., medical patients, and moreparticularly to spectrophotometric oximetry, by transmitting selectedwavelengths (spectra) of light into a given area of the test subject,receiving the resulting light as it leaves the subject at predeterminedlocations, and analyzing the received light to determine the desiredconstituent data based on the spectral absorption which has occurred,from which metabolic information such as blood oxygen saturation may becomputed for the particular volume of tissue through which the lightspectra have passed.

A considerable amount of scientific data and writings, as well as priorpatents, now exist which is/are based on research and clinical studiesdone in the above-noted area of investigation, validating the underlyingtechnology and describing or commenting on various attributes andproposed or actual applications of such technology. One such applicationand field of use is the widespread clinical usage of pulse oximeters asof the present point in time, which typically utilize sensors applied tobody extremities such as fingers, toes, earlobes, etc., where arterialvasculature is in close proximity, from which arterial hemoglobinoxygenation may be determined non-invasively. A further and importantextension of such technology is disclosed and discussed in U.S. Pat. No.5,902,235, which is related to and commonly owned with the presentapplication and directed to a non-invasive spectrophotometric cerebraloximeter, by which blood oxygen saturation in the brain may benon-invasively determined through the use of an optical sensor havinglight emitters and detectors that is applied to the forehead of thepatient. Earlier patents commonly owned with the '235 patent and thepresent one pertaining to various attributes of and applications for theunderlying technology include U.S. Pat. Nos. 5,139,025; 5,217,013;5,465,714; 5,482,034; and 5,584,296.

The cerebral oximeter of the aforementioned '235 patent has proved to bean effective and highly desirable clinical instrument, since it providesuniquely important medical information with respect to brain condition(hemoglobin oxygen saturation within the brain, which is directlyindicative of the single most basic and important life parameter, i.e.brain vitality). This information was not previously available, despiteits great importance, since there really is no detectable arterial pulsewithin brain tissue itself with respect to which pulse oximetry could beutilized even if it could be effectively utilized in such an interiorlocation (which is very doubtful), and this determination thereforerequires a substantially different kind of apparatus and determinationanalysis. In addition, there are a number of uniquely complicatingfactors, including the fact that there is both arterial and venousvasculature present in the skin and underlying tissue through which theexamining light spectra must pass during both entry to and exit from thebrain, and this would distort and/or obscure the brain examination dataif excluded in some way. Furthermore, the overall blood supply withinthe skull and the brain itself consists of a composite of arterial,venous, and capillary blood, as well as some pooled blood, and each ofthese are differently oxygenated. In addition, the absorption andscatter effects on the examination light spectra are much greater in thebrain and its environment than in ordinary tissue, and this tends toresult in extremely low-level electrical signal outputs from thedetectors for analysis, producing difficult signal-to-noise problems.

Notwithstanding these and other such problems, the cerebral oximeterembodying the technology of the aforementioned issued patents (nowavailable commercially from Somanetics Corporation, of Troy, Mich.) hasprovided a new type of clinical instrument by which new information hasbeen gained relative to the operation and functioning of the humanbrain, particularly during surgical procedures and/or injury or trauma,and this has yielded greater insight into the functioning and state ofthe brain during such conditions. This insight and knowledge has greatlyassisted surgeons performing such relatively extreme procedures ascarotid endarterectomy, brain surgery, and other complex procedures,including open-heart surgery, etc. and has led to a greaterunderstanding and awareness of conditions and effects attributable tothe hemispheric structure of the human brain, including the functionalinter-relationship of the two cerebral hemispheres, which are subtlyinterconnected from the standpoint of blood perfusion as well as that ofelectrical impulses and impulse transfer.

BRIEF SUMMARY OF INVENTION

The present invention results from the new insights into and increasedunderstanding of the human brain referred to in the preceding paragraph,and provides a methodology and apparatus for separately (and preferablysimultaneously) sensing and quantitatively determining brain oxygenationat a plurality of specifically different locations or regions of thebrain, particularly during surgical or other such traumatic conditions,and visually displaying such determinations in a directly comparativemanner. In a larger sense, the invention may also be used to monitoroxygenation (or other such metabolite concentrations or parameters) inother organs or at other body locations, where mere arterial pulseoximetry is a far too general and imprecise examination technique.

Further, and of considerable moment, the invention provides a method andapparatus for making and displaying determinations of internal metabolicsubstance, as referred to in the preceding paragraph, at a plurality ofparticular and differing sites, and doing so on a substantiallysimultaneous and continuing basis, as well as displaying thedeterminations for each such site in a directly comparative manner, forimmediate assessment by the surgeon or other attending clinician, on areal-time basis, for direct support and guidance during surgery or othersuch course of treatment.

In a more particular sense, the invention provides a method andapparatus for spectrophotometric in vivo monitoring of blood metabolitessuch as hemoglobin oxygen concentration in any of a preselectedplurality of different regions of the same test subject and on acontinuing and substantially instantaneous basis, by applying aplurality of spectrophotometric sensors. In a more particular sense, theinvention provides a method and apparatus for spectrophotometric in vivomonitoring of blood metabolites such as hemoglobin oxygen concentrationin any of a preselected plurality of different regions of the same testsubject and on a continuing and substantially instantaneous basis, byapplying a plurality of spectrophotometric sensors to the test subjectat each of a corresponding plurality of testing sites, coupling eachsuch sensor to a control and processing station, operating each suchsensor to spectrophotometrically irradiate a particular region withinthe test subject associated with that sensor, detecting and receivingthe light energy resulting from such spectrophotometric irradiation foreach such region, conveying signals corresponding to the light energy soreceived to the control and processing station, analyzing the conveyedsignals to determine preselected blood metabolite data, and displayingthe data so obtained from each of a plurality of such testing sites andfor each of a plurality of such regions, in a region-comparative manner.

The foregoing principal aspects and features of the invention willbecome better understood upon review of the ensuing specification andthe attached drawings, describing and illustrating preferred embodimentsof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial representation of a patient on whom apparatus inaccordance with the invention is being used;

FIG. 2 is a fragmentary plan view of a typical sensor used in accordancewith the invention;

FIG. 3 is an enlarged, fragmentary, pictorial cross-sectional view of ahuman cranium, showing the sensors of FIG. 2 applied and in place,generally illustrating both structural and functional aspects of theinvention;

FIG. 4 is a front view of a typical control and processing unit for usein the invention, illustrating a preferred display of data determined inaccordance with the invention;

FIGS. 5, 6, and 7 are graphs representing data displays obtained inaccordance with the invention which represent actual surgical procedureresults from actual patients;

FIG. 8 is a pictorialized cross-sectional view representing a testsubject on which a multiplicity of sensors are placed in sequence,further illustrating the multi-channel capability of the presentinvention;

FIG. 9 is a schematic block diagram generally illustrating thecomponentry and system organization representative of a typicalimplementation of the invention; and

FIG. 10 is a pictorialized cross-sectional view similar to FIG. 8, butstill further illustrating the multi-channel capability of the presentinvention.; and

FIG. 11 is the pictorialized cross-sectional view of FIG. 10 withannotations identifying particular optical elements, as well as theirspacing and relationships;

FIG. 12 is an enlarged view of the cross-sectional view of FIG. 11 thatdepicts the spacing and relationships for some of the identified opticalelements;

FIG. 13 is the pictorialized cross-sectional view of FIG. 10 withannotations identifying particular optical elements, as well as theirspacing and mean paths;

FIG. 14 is an enlarged view of the cross-sectional view of FIG. 13 thatdepicts the spacing and mean paths for some of the identified opticalelements; and

FIG. 15 is the pictorialized cross-sectional view of FIG. 10 withannotations identifying particular optical elements, as well as theirspacing and relationships.

DESCRIPTION OF PREFERRED EMBODIMENT

FIG. 1 depicts an illustrative patient 10 on whom an instrument 12 inaccordance with the present invention is being employed. As illustrated,the forehead 14 of patient 10 has a pair of sensors 16, 116 secured toit in a bilateral configuration, i.e., one such sensor on each side ofthe forehead, where each may monitor a different brain hermisphere. Eachof the sensors 16, 116 is connected to a processor and display unit 20which provides a central control and processing station (sometimeshereinafter, referred to as the “oximeter”) by a correspondingelectrical cable 16A, 116A, which join one another at a dual-channelcoupler/pre-amp 18, 118 and then (preferably) proceed to the control andprocessor 20 as an integrated, multiple-conductor cable 22. As will beunderstood, the electrical cables just noted include individualconductors for energizing light emitters and operating the related lightdetectors contained in sensors 16, 116, all as referred to furtherhereinafter and explained in detail in the various prior patents.

The general nature of a typical structure and arrangement for thesensors 16,116 (which are identical in nature and which may if desiredbe incorporated into a single physical unit) is illustrated in FIG. 2,and comprises the subject matter of certain of the earlier patents, inparticular U.S. Pat. Nos. 5,465,714; 5,482,034; 5,584,296; and5,795,292, wherein the structure and componentry of preferred sensorsare set forth in detail. For present purposes, it is sufficient to notethat the sensors 16, 116 include an electrically actuated light source24 for emitting the selected examination spectra (e.g., two or morenarrow-bandwidth LEDs, whose center output wavelengths correspond to theselected examination spectra), together with a pair of light detectors26, 28 (e.g., photodiodes) which are preferably located at selected andmutually different distances from the source 24. These electro-optical(i.e., “optode”) components are precisely positioned upon and securedto, or within, a sensor body having a foam or other such soft andconformable outer layer which is adhesively secured to the forehead (orother desired anatomical portion) of the patient 10, as generallyillustrated in FIG. 1, and individual electrical conductors in cables16A, 116A provide operating power to the sources 24 while others carryoutput signals from the detectors 26, 28, which are representative ofdetected light intensities received at the respective detector locationsand must be conveyed to the processor unit 20, where processing takesplace.

FIG. 3 generally illustrates, by way of a pictorialized cross-sectionalview, the sensors 16, 116 in place upon the forehead 14 of the patient12. As illustrated in this figure, the cranial structure of patient 12generally comprises an outer layer of skin 30, an inner layer of tissue32, and the frontal shell 34 of the skull, which is of course bone.

Inside the skull 34 is the Periosteal Dura Mater, designated by thenumeral 36, and inside that is the brain tissue 38 itself, which iscomprised of two distinct hemispheres 38′, 38″ that are separated at thecenter of the forehead inwardly of the superior sagital sinus by a thin,inwardly-projecting portion 36a of the Dura 36. Thus, in the arrangementillustrated in FIG. 3, sensor 16 accesses and examines brain hemisphere38″, while sensor 116 does the same to brain hemisphere 38′.

As explained at length in various of the above-identified prior patents,the preferred configuration of sensors 16, 116 includes both a “near”detector 26, which principally receives light from source 24 whose meanpath length is primarily confined to the layers of skin, tissue, skull,etc., outside brain 38, and a “far” detector 28, which receives lightspectra that have followed a longer mean path length and traversed asubstantial amount of brain tissue in addition to the bone and tissuetraversed by the “near” detector 26. Accordingly, by appropriatelydifferentiating the information from the “near” (or “shallow”) detector26 (which may be considered a first data set) from information obtainedfrom the “far” (or “deep”) detector 28 (providing a second such dataset), a resultant may be obtained which principally characterizesconditions within the brain tissue itself, without effects attributableto the overlying adjacent tissue, etc. This enables the apparatus toobtain metabolic information on a selective basis, for particularregions within the test subject, and by spectral analysis of thisresultant information, employing appropriate extinction coefficients,etc. (as set forth in certain of the above-identified patents), anumerical value, or relative quantified value, may be obtained whichcharacterizes metabolites or other metabolic data (e.g., the hemoglobinoxygen saturation) within only the particular region or volume of tissueactually examined, i.e., the region or zone generally defined by thecurved mean path extending from source 24 to the “far” or “deep”detector 28, and between this path and the outer periphery of the testsubject but excluding the analogous region or zone defined by the meanpath extending from source 24 to “near” detector 26. As will beunderstood, particularly in view of Applicants' above-identified priorpatents as well as is explained further hereinafter, this data analysiscarried out by the “control and processing unit” 20 is accomplished byuse if an appropriately programmed digital computer, as is now known bythose skilled in the art (exemplified in particular by the Somanetics®model 4100 cerebral oximeter).

The present invention takes advantage of the primarily regional oxygensaturation value produced by each of the two (or more) sensors 16, 116,together with the natural hemispheric structure of brain 38, by use of acomparative dual or other multi-channel examination paradigm that in thepreferred embodiment or principal example set forth herein provides aseparate but preferably comparatively displayed oxygen saturation valuefor each of the two brain hemispheres 38′, 38″. Of course, it will beunderstood that each such regional index or value of oxygen saturationis actually representative of the particular region within a hemisphereactually subjected to the examining light spectra, and while each suchregional value may reasonably be assumed to be generally representativeof the entire brain hemisphere in which it is located, and thereforuseful in showing and contrasting the differing conditions between thetwo such hemispheres of the brain 38, the specific nature andunderstanding of these hemispheric interrelationships and ofinterrelationships between other and different possible sensor locationsrelative to each different hemisphere 38′, 38″ are not believed to befully known and appreciated as of yet. Consequently, it may be useful oradvantageous in at least some cases, and perhaps in many, to employ amore extensive distribution and array of sensors and correspondinginputs to the oximeter 20, such as is illustrated for example in FIG. 8.

Thus, as seen in FIG. 8, a more extensive array of sensors 16, 116, 216,etc., may be deployed around the entire circumference of the head orother such patient extremity, for example, each such sensor sampling adifferent regional area of each brain hemisphere or other such organ ortest site and outputting corresponding data which may be contrasted invarious ways with the analogous data obtained from the other suchsensors for other test site regions. In this regard, it will beappreciated that the extent of each such regional area subjected toexamination is a function of a number of different factors, particularlyincluding the distance between the emitter or source 24 and detectors26, 28 of each such set and the amount of light intensity which isutilized, the greater the emitter/sensor distance and correspondinglight intensity, the greater the area effectively traversed by theexamining light spectra and the larger the size of the “region” whoseoximetric or other metabolic value is being determined.

It may also be possible to use only a single source position and employa series of mutually spaced detector sets, or individual detectors,disposed at various selected distances from the single source around allor a portion of the perimeter of the subject. Each such single sourcewould actually illuminate the entire brain since the photons sointroduced would scatter throughout the interior of the skull (eventhough being subject to increased absorption as a function of distancetraversed), and each such emitter/detector pair (including long-rangepairs) could produce information characterizing deeper interior regionsthan is true of the arrays illustrated in FIGS. 3 and 8, for example. Ofcourse, the smaller-region arrays shown in these figures are desirablein many instances, for a number of reasons. For example, the comparativeanalysis of information corresponding to a number of differing suchregions, as represented by the array of FIG. 8, lends itself readily tovery meaningful comparative displays, including for examplecomputer-produced mapping displays which (preferably by use of differingcolors and a color monitor screen) could be used to present an ongoingreal-time model which would illustrate blood or even tissue oxygenationstate around the inside perimeter of and for an appreciable distancewithin a given anatomical area or part. The multiple detector outputsfrom such a single-source arrangement, on the other hand, would containinformation relative to regions or areas deep within the brain, andmight enable the determination of rSO₂ values (or other parameters) fordeep internal regions as well as the production of whole-brain mapping,by differentially or additively combining the outputs from variousselected detectors located at particular points.

The dual or bilateral examination arrangement depicted in FIGS. 1 and 3will provide the highly useful comparative display formats illustratedin FIGS. 4, 5, 6, and 7 (as well as on the face of the oximeter 20 shownat the right in FIG. 1), for example. In the arrangement shown in FIGS.1 and 4, each sensor output is separately processed to provide aparticular regional oxygen saturation value, and these regional valuesare separately displayed on a video screen 40 as both a numeric or othersuch quantified value, constituting a basically instantaneous real-timevalue, and as a point in a graphical plot 42, 44, representing asuccession of such values taken over time. As illustrated, the plots orgraphs 42, 44 may advantageously be disposed one above the other indirect alignment, for convenient examination and comparison. While theinstantaneous numeric displays will almost always be found useful anddesirable, particularly when arranged in the directly adjacent andimmediately comparable manner illustrated, the graphical trace displays42, 44 directly show the ongoing trend, and do so in a contrasting,comparative manner, as well as showing the actual or relative values,and thus are also highly useful.

Graphic displays 42, 44 may also advantageously be arranged in the formshown in FIGS. 5, 6, and 7, in which the two such individual traces aredirectly superimposed upon one another, for more immediate and readilyapparent comparison and contrast. Each of the examples shown in FIGS. 5,6, and 7 does in fact represent the record from an actual surgicalprocedure in which the present invention was utilized, and in each ofthese the vertical axis (labeled rSO₂) is indicative of regional oxygensaturation values which have been determined, while the horizontal axisis, as labeled, “real time,” i.e., ongoing clock time during thesurgical procedure involved. The trace from the “left” sensor (number 16as shown in FIGS. 1 and 3), designated by the numeral 42 forconvenience, is shown in solid lines in these graphs, whereas the trace44 from the right-hand sensor 116 is shown in dashed lines. The sensorsmay be placed on any region of their respective test areas (e.g., brainhemispheres) provided that any underlying hair is first removed, sincehair is basically opaque to the applied light spectra and thus greatlyreduces the amount of light energy actually introduced to the underlyingtissue, etc.

With further reference to FIGS. 5, 6, and 7, and also inferentially toFIG. 4, it will be seen that at certain times, (e.g., the beginning andend of each procedure, when the patient's condition is at leastrelatively normal) there is a certain amount of direct correspondencebetween the two different hemispheric traces 42, 44, and that in atleast these time increments the shape of the two traces is reasonablysymmetrical and convergent. An idealized such normal result is shown inFIG. 1, wherein both the numeric values and the curves are basically thesame. In each of the procedures shown in FIGS. 5, 6, and 7, however,there are times when the detected regional cerebral oxygen saturationdiffers markedly from one brain hemisphere to the other. This isparticularly noticeable in FIG. 6, in which it may be observed that theleft hand trace 42 is at times only about one half the height (i.e.,value) of the right hand trace 44, reaching a minimal value in theneighborhood of about 35% slightly before real time point 12:21 ascompared to the initial level, at time 10:50-11:00, of more than 75%,which is approximately the level of saturation in the right hemisphereat the 12:21 time just noted, when the oxygenation of the lefthemisphere had decreased to approximately 35%.

As will be understood, the various differences in cerebral bloodoxygenation shown by the superimposed traces of FIGS. 5, 6, and 7 occuras a result of measures taken during the corresponding surgicalprocedures, which in these cases are carotid endarterectomies and/orcoronary artery bypass graft (CABG), which are sometimes undertaken as acontinuing sequence. In the illustrated examples, FIG. 5 represents asequential carotid endarterectomy and hypothermic CABG, in which thevertical lines along the time axis characterize certain events duringsurgery, i.e., index line 46 represents the time of the carotid arterialincision, line 48 represent the time the arterial clamp was applied andthe shunt opened (resulting in reduced arterial blood flow to the leftbrain hemisphere), index line 50 represents a time shortly after theshunt was removed and the clamp taken off, and the area from about realtime 17:43 to the end of the graph was when the hypothermic brainsurgery actually took place, the lowest point (just prior to time 18:23)occurring when the heart-lung machine pump was turned on, and theindices at time 19:43 and 20:23 generally show the time for bloodrewarming and pump off, respectively. While illustrative and perhapsenlightening, it is not considered necessary to give the specifics ofthe surgical procedures portrayed by the graphical presentations ofFIGS. 6 and 7, although it may be noted that the procedure of FIG. 6 wasa carotid endarterectomy of the left side and that of FIG. 7 was asimilar endarterectomy on the right side of a different patient.Sufficient to say that these graphs represent other such surgicalprocedures and show comparable states of differing hemisphericoxygenation.

The importance and value of the information provided in accordance withthe present invention is believed self-apparent from the foregoing,particularly the graphical presentations of and comments provided withrespect to FIGS. 5, 6, and 7. Prior to the advent of the presentinvention, no such comparative or hemispheric-specific information wasavailable to the surgeon, who did not in fact have any quantified oraccurately representative data to illustrate the prevailing hemisphericbrain oxygenation conditions during a surgery. Thus, even the use of asingle such sensor (16, 116) on the side of the brain on which aprocedure is to be done is highly useful and, as of the present time,rapidly being recognized as essential. Of course, it is considerablymore useful to have at least the bilateral array illustrated in FIG. 1,to provide comparative data such as that seen in FIGS. 4-7 inclusive.

FIG. 9 is a schematic block diagram generally illustrating thecomponentry and system organization making up a typical implementationof the invention, as shown pictorially in FIG. 1 (to which reference isalso made). As shown in FIG. 9, the oximeter 20 comprises a digitalcomputer 50 which provides a central processing unit, with a processor,data buffers, and timing signal generation for the system, together witha keypad interface (shown along the bottom of the unit 20 in FIG. 1),display generator and display 40 (preferably implemented by use of aflat electro-luminescent unit, at least in applications where a sharpmonochromatic display is sufficient), as well as an audible alarm 52including a speaker, and a data output interface 54 by which thecomputer may be interconnected to a remote personal computer, diskdrive, printer, or the like for downloading data, etc.

As also shown in FIG. 9, each of the sensors 16, 116 (and/or others, inthe multi-site configuration illustrated in FIG. 8) receives timingsignals from the CPU 50 and is coupled to an LED excitation currentsource (56,156) which drives the emitters 24 of each sensor. The analogoutput signals from the detectors (photodiodes) 26, 28 of each sensorare conveyed to the coupler/pre-amp 18, 118 for signal conditioning(filtering and amplification), under the control of additional timingsignals from the CPU. Following that, these signals undergo A-to-Dconversion and synchronization (for synchronized demodulation, as notedhereinafter), also under the control of timing signals from CPU 50, andthey are then coupled to the CPU for computation of regional oxygensaturation rSO₂ data, storage of the computed data, and display thereof,preferably in the format discussed above in conjunction with FIGS. 4, 5,6, and 7. As will be apparent, each sensor (16, 116, etc.) preferablyhas its own signal-processing circuitry (pre-amp, etc.) upstream of CPU50, and each such sensor circuit is preferably the same.

While implementation of a system such as that shown in FIG. 9 is as ageneral matter well within the general skill of the art once the natureand purpose of the system and the basic requirements of its components,together with the overall operation (as set forth above and hereinafter)have become known, at least certain aspects of the preferred such systemimplementation are as follows. First, it is preferable that the lightemitters 24 (i.e., LEDs) of each of the different sensors 16, 116 etc.,be driven out-of-phase, sequentially and alternatingly with one another(i.e., only a single such LED or other emitter being driven during thesame time interval, and the emitters on the respective different sensorsare alternatingly actuated, so as to ensure that the detectors 26, 28 ofthe particular sensor 16, 116 then being actuated receive only resultantlight spectra emanating from a particular emitter located on thatparticular sensor, and no cross-talk between sensors takes place (eventhough significant levels of cross-talk are unlikely in any event due tothe substantial attenuation of light intensity as it passes throughtissue, which is on the order of about ten times for each centimeter ofoptical path length through tissue). Further, it is desirable tocarefully window the “on” time of the detectors 26, 28 so that each isonly active during a selected minor portion (for example, 10% or less)of the time that the related emitter is activated (and, preferably,during the center part of each emitter actuation period). Of course,under computer control such accurate and intricate timing is readilyaccomplished, and in addition, the overall process may be carried on ata very fast rate.

In a multi-site (multiple sensor) system, such as that shown in FIG. 8,the preferred implementation and system operation would also be inaccordance with that shown in FIG. 9, and the foregoing commentsregarding system performance, data sampling, etc., would also apply,although there would of course be a greater number of sensors and sensorcircuit branches interfacing with computer 50. The same would also bebasically true of a single-source multi-site detector configuration orgrouping such as that referred to above, taking into consideration thefact that the detectors would not necessarily be grouped in specific ordedicated “near-far” pairs and bearing in mind that one or moredetectors located nearer a source than another detector, or detectors,located further from the source could be paired with or otherwise deemeda “near” detector relative to any such farther detector. In any suchmultiple-site configuration, it may be advantageous to implement aprioritized sequential emitter actuation and data detection timingformat, in which more than one emitter may be operated at the same time,or some particular operational sequence is followed, with appropriatesignal timing and buffering, particularly if signal cross-talk is not amatter of serious consideration due to the particular circumstancesinvolved (detector location, size and nature of test subject,physiology, signal strength, etc.). As illustrated in FIG. 10, amulti-sensor or multiple sector-emitter array may be so operated, byusing a number of different emitter-detector pair groupings, with somedetectors used in conjunction with a series of different emitters tomonitor a number of differing internal sectors or regions.

A system as described above may readily be implemented to obtain on theorder of about fifteen data samples per second even with the minimaldetector “on” time noted, and a further point to note is that thepreferred processing involves windowing of the detector “on” time sothat data samples are taken alternatingly during times when the emittersare actuated and the ensuing time when they are not actuated (i.e.,“dark time”), so that the applicable background signal level may becomputed and utilized in analyzing the data taken during the emitter“on” time. Other features of the preferred processing include the takingof a fairly large number (e.g., 50) of data samples during emitter “on”time within a period of not more than about five seconds, and processingthat group of signals to obtain an average from which each updated rSO₂value is computed, whereby the numeric value displayed on the videoscreen 40 is updated each five seconds (or less). This progression ofcomputed values is preferably stored in computer memory over the entirelength of the surgical procedure involved, and used to generate thegraphical traces 42, 44 on a time-related basis as discussed above.Preferably, non-volatile memory is utilized so that this data will notbe readily lost, and may in fact be downloaded at a convenient timethrough the data output interface 54 of CPU 50 noted above in connectionwith FIG. 9.

As shown in FIGS. 11 and 12, a first emitter 624, a second emitter 626,a first detector 628, and a second detector 630 are placed over a firsttissue region 632. The first emitter 624 is adapted to emit a firstlight into the first tissue region 632 and the second emitter 626 isadapted to emit a second light into the first tissue region 632. Thefirst detector 628 is located a first distance 634, also referred to asthe first line 634, from the first emitter 624 and is located a seconddistance 636, also referred to as the second line 636, from the secondemitter 626. As shown in these figures, the second distance 636 isgreater than the first distance 634. The second detector 630 is locateda third distance 638, also referred to as the third line 638, from thefirst emitter 624 and is located a fourth distance 640, also referred toas the fourth line 640, from the second emitter 626. As shown in thesefigures, the fourth distance 640 is less than the third distance 638.The first emitter 624 is closer to the first detector 628 than thesecond detector 630, and the second emitter 626 is closer to the seconddetector 630 than the first detector 628. The third distance 638 islonger than the first distance 634 and is longer than the fourthdistance 640. The second distance 636 is approximately equal to thethird distance 638. The first distance 634 is approximately equal to thefourth distance 640.

As further shown in FIGS. 11 and 12, the first emitter 624, the secondemitter 626, the first detector 628 and the second detector 630 arealigned within the cross-sectional plane. In addition, the second line636 defined between the center of the first detector 628 and the centerof the second emitter 626 partially overlaps with the third line 638defined between the center of the second detector 630 and the center ofthe first emitter 624.

Referring now to FIG. 11, a third emitter 724, a fourth emitter 726, athird detector 728, and a fourth detector 730 are placed over a secondtissue region 732. The third emitter 724 is adapted to emit a thirdlight into the second tissue region 732 and the fourth emitter 726 isadapted to emit a fourth light into the second tissue region 732. Thethird detector 728 is located a fifth distance 734, also referred to asthe fifth line 734, from the third emitter 724 and is located a sixthdistance 736, also referred to as the sixth line 736, from the secondemitter 726. The second detector 730 is located a seventh distance 738,also referred to as the seventh line 738, from the third emitter 724 andis located an eighth distance 740, also referred to as the eighth line740, from the fourth emitter 726. As also shown in FIG. 11, the thirdemitter 724 is closer to the third detector 728 than the fourth detector730, and the fourth emitter 726 is closer to the fourth detector 730than the third detector 728. The fifth distance 734 is less than theseventh distance 738. The eighth distance 740 is less than the sixthdistance 736.

As shown in FIGS. 13 and 14, the first detector 628 is adapted to detectthe first light propagated over a first mean path 664 through the firsttissue region 632 and to detect the second light propagated over asecond mean path 666 through the first tissue region 632. The secondmean path 666 has a length 667 greater than a length 665 of the firstmean path 664. The second detector 630 is adapted to detect the firstlight propagated over a third mean path 668 through the first tissueregion 632 and is adapted to detect the second light propagated over afourth mean path 670 through the first tissue region 632. The fourthmean path 670 has a length 671 less than the length 669 of the thirdmean path 668. The length 665 of the first mean path 664 issubstantially equivalent to the length 671 of the fourth mean path 670and the length 669 of the third mean path 668 is substantiallyequivalent to the length 667 of the second mean path 666. The length 665of the first mean path 664 is less than the length 669 of the third meanpath 668 and the length 671 of the fourth mean path 670 is less than thelength 667 of the second mean path 666. The second mean path 666 and thethird mean 668 path overlap at a location 672 below a tissue surface ofthe tissue region 632. In addition, along a line 674 orthogonal to thesurface of the tissue between the first detector 628 and the seconddetector 630, the third mean path 668 lies farther from the tissuesurface than the second mean path 666. The second mean path 666 liessubstantially as far from a tissue surface as the third mean path 668 atapproximately a midpoint 676 between the first detector 628 and thesecond detector 630.

As further shown in FIGS. 13 and 14, the first emitter 624 and the firstdetector 628 form a first near coupling. The second detector 630 islocated farther from the first emitter 624 than the first detector 628to form a first far coupling. The second emitter 626 and the firstdetector 628 form a second far coupling. The second detector 630 islocated closer to the second emitter 626 than the first detector 628 toform a second near coupling. The first emitter 624 is adapted totransmit the first light along the first mean path 664 through a firstsection 680 of the first tissue region 632. The second emitter 626 isadapted to transmit the second light along the second mean path 666through the first section 680 of the first tissue region 632 and thefourth mean path 670 through a second section 682 of the first tissueregion 632. The first emitter is adapted to transmit the first lightalong the third mean path 668 through the second section 682 of thefirst tissue region 632. The first emitter 624 and the second emitter626 are further adapted to transmit the first light and the second lightalong the third mean path 668 and second mean path 666, respectively,through a third section 684 of the first tissue region 632 and totransmit the first light and the second light along the first mean path664 and the fourth mean path 670, respectively, that substantially avoidthe third section 684 of the first tissue region 632.

As shown in FIG. 13, the third detector 728 is adapted to detect thethird light propagated over a fifth mean path 764 through the secondtissue region 732. The third detector 728 is adapted to detect thefourth light propagated over a sixth mean path 766 through the secondtissue region 732. The fourth detector 730 is adapted to detect thethird light propagated over a seventh mean path 768 through the secondtissue region 732. The fourth detector 730 is adapted to detect thefourth light propagated over an eighth mean path 770 through the secondtissue region 732. The length 769 of the seventh mean path 768 isgreater than the length 765 of the fifth mean path 764 and the length767 of the sixth mean path 766 is greater than the length 771 of theeighth mean path 770.

As shown in FIG. 15, a first transmitter 724 (previously referred to asthe third emitter 724 during the discussion of FIGS. 11 and 13 above), afirst detector 826, a second detector 828, and a third detector 830 areplaced over a first region of tissue 732 (previously referred to as thesecond tissue region 732 during the discussion of FIGS. 11 and 13above). The first transmitter 724 is adapted to transmit light into thefirst region of tissue 732. The first detector 826 forms a near detectorgrouping with the first transmitter 724. The second detector 828 and thethird detector 830 are located farther from the first transmitter 724than the first detector 826 to form far detector groupings. As alsoshown in FIG. 15, a line 840 passing through a midpoint of the firsttransmitter 724 and a midpoint of the first detector 826 is spaced apartfrom a midpoint of the second detector 828 and a midpoint of the thirddetector 830. In addition, the line 840 defined between a center of thefirst transmitter 724 and the center of the first detector 826 forms anacute angle 842 with a line 844 defined between the center of thetransmitter 724 and a center of the second detector 828. The line 840defined between the center of the first transmitter 724 and the centerof the first detector 826 forms a second acute angle 846 with a line 848defined between the center of the transmitter 724 and a center of thethird detector 830, with the second acute angle 846 substantiallysimilar to the first acute angle 842.

As further shown in FIG. 15, a second transmitter 624 (previouslyreferred to as the first emitter 624 during the discussion of FIGS.11-14 above), a fourth detector 628 (previously referred to as the firstdetector 628 during the discussion of FIGS. 11-14 above), a fifthdetector 928, and a sixth detector 930 are placed over a second regionof tissue 632 (previously referred to as the first tissue region 632during the discussion of FIGS. 11-14 above). The fourth detector 628forms a near detector grouping with the second transmitter 624. Thefifth detector 928 and the sixth detector 930 are each located fartherfrom the second transmitter 624 than the fourth detector 628 to form fardetector groupings. As shown in FIG. 15, the distance 940 between thefirst transmitter 724 and the first detector 826 is approximately equalto the distance 942 between the second transmitter 624 and the fourthdetector 628.

As will be understood, the foregoing disclosure and attached drawingsare directed to a single preferred embodiment of the invention forpurposes of illustration; however, it should be understood thatvariations and modifications of this particular embodiment may welloccur to those skilled in the art after considering this disclosure, andthat all such variations etc., should be considered an integral part ofthe underlying invention, especially in regard to particular shapes,configurations, component choices and variations in structural andsystem features. Accordingly, it is to be understood that the particularcomponents and structures, etc. shown in the drawings and describedabove are merely for illustrative purposes and should not be used tolimit the scope of the invention, which is defined by the followingclaims as interpreted according to the principles of patent law,including the doctrine of equivalents.

The invention claimed is:
 1. A method for comparative spectrophotometricin vivo monitoring and display of selected blood metabolites present ina plurality of different internal regions of the same test subject on acontinuing and substantially concurrent basis, comprising the steps of:applying separate spectrophotometric sensors to a test subject at eachof a plurality of separate testing sites and coupling each of saidsensors to a control and processing station; operating a selected numberof said sensors on a substantially concurrent basis tospectrophotometrically irradiate at least two separate internal regionsof the test subject during a common time interval, each of said regionsbeing associated with a different of said testing sites; separatelydetecting and receiving light energy resulting from saidspectrophotometric irradiation for each of said at least two separateinternal regions, and conveying separate sets of signals to said controland processing station which correspond to the separately detected lightenergy from said at least two separate internal regions; separately andconcurrently analyzing said conveyed separate sets of signals toseparately determine quantified data representative of a bloodmetabolite in each of said at least two separate internal regions; andconcurrently visually displaying said separately determined quantifieddata for each of said at least two separate internal regions for directconcurrent mutual comparison, wherein said sensors are applied to a headof the test subject and are used to monitor two mutually separateregions within a brain of the test subject.
 2. The method of claim 1,wherein said step of analyzing comprises quantitative determination ofblood oxygenation levels within each of said at least two separateinternal regions.
 3. The method of claim 2, wherein said analyzing stepincludes producing separate quantitative value determinations forhemoglobin oxygen saturation for each of said at least two separateinternal regions.
 4. The method of claim 3, wherein said analyzing stepincludes production of ongoing graphical traces representing a pluralityof said quantitative value determinations made at successive points intime.
 5. The method of claim 4 including the step of visually displayinga plurality of said graphical traces at substantially the same time andin predetermined relationship to one another to facilitate rapid andaccurate visual comparison.
 6. The method of claim 5, including the stepof visually displaying a plurality of said quantitative valuedeterminations at substantially the same time and in predeterminedrelationship to one another to facilitate rapid and accurate visualcomparison.
 7. The method of claim 3, including the step of visuallydisplaying a plurality of said quantitative value determinations atsubstantially the same time and in predetermined relationship to oneanother to facilitate rapid and accurate visual comparison.
 8. Themethod of claim 1, wherein said metabolite comprises hemoglobin oxygen.9. The method of claim 1, wherein said sensors are positioned inlocations proximate to different brain hemispheres and said two mutuallyseparate regions are located in a different brain hemisphere.
 10. Themethod of claim 9, wherein said metabolite comprises cerebral bloodhemoglobin oxygenation.
 11. An apparatus for concurrent comparativespectrophotometric in vivo monitoring of selected blood metabolitespresent in each of a plurality of different internal regions on acontinuing basis, comprising: a plurality of spectrophotometric sensors,each attachable to a test subject at different test locations andadapted to separately but concurrently spectrophotometrically irradiateat least two different internal regions within the test subjectassociated with each of said test locations; a controller and circuitrycoupling each of said sensors to said controller for separately andindividually but concurrently operating certain of said sensors tospectrophotometrically irradiate each of said different internal regionswithin the test subject associated with each of said test locations;said sensors each further adapted to receive light energy resulting fromthe separate spectrophotometric irradiation of said sensors' associatedone of said at least two different internal regions on a substantiallyconcurrent basis with other said sensors, and to produce separatesignals corresponding to the light energy received, said circuitryacting to convey said separate signals to said controller for separateanalytic processing; said controller adapted to analytically processsaid conveyed signals separately and determine separate quantified bloodmetabolite data therefrom for each of said sensors' and said sensorsassociated one of said at least two different internal regions; and avisual display coupled to said controller and adapted to separately butconcurrently display the quantified blood metabolite data determined foreach of said sensors in a mutually-comparative manner, wherein saidsensors are adapted to be applied to a head of the test subject and tomonitor a brain of the test subject.
 12. The apparatus of claim 11,wherein said controller is adapted to analyze said data toquantitatively determine blood oxygenation within said at least twodifferent internal regions.
 13. The apparatus of claim 12, wherein saidcontroller is adapted to produce separate numeric value designations forhemoglobin oxygen saturation for said at least two different internalregions.
 14. The apparatus of claim 13, wherein said controller and saiddisplay are adapted to produce ongoing graphical traces representing aplurality of said numeric value designations for the same region takenover a period of time.
 15. The apparatus of claim 14, wherein saidcontroller and said display are adapted to visually display at least twoof said graphical traces on a substantially concurrent basis and inpredetermined relationship to one another to facilitate rapid andaccurate visual comparison.
 16. The apparatus of claim 15, wherein saidcontroller and said display are adapted to visually display at least twoof said numeric value designations as well as at least two of saidgraphical traces on a substantially concurrent basis and in proximity toone another to facilitate rapid and accurate visual comparison.
 17. Theapparatus of claim 13, wherein said controller and said display areadapted to visually display at least two of said numeric valuedesignations on a substantially concurrent basis and in predeterminedrelationship to one another to facilitate rapid and accurate visualcomparison.
 18. The apparatus of claim 11, wherein said sensors areadapted to provide signals to said controller which comprise at leasttwo separate data sets that cooperatively define at least portions of aparticular area within a given one of said at least two differentinternal regions.
 19. The apparatus of claim 18, wherein said data setsprovided by said sensors include a first set characterizing a first partof said particular area and a second set characterizing a second part ofsaid particular area.
 20. The apparatus of claim 19, wherein said secondpart of said particular area characterized by said second set includesat least part of said first part of said area.
 21. The apparatus ofclaim 11, wherein said controller is adapted to determine bloodoxygenation saturation in said brain.
 22. The apparatus of claim 11,wherein at least two of said sensors are adapted to be positioned inlocations associated with mutually different hemispheres of the brainand each of said sensors is operable to separately monitor at leastportions of each of said different hemispheres.
 23. The apparatus ofclaim 22, wherein said controller is adapted to determine cerebral bloodoxygenation saturation within each of said different hemispheres. 24.The apparatus of claim 22, wherein said sensors are adapted to providesignals to said controller which comprise at least two data sets thatcooperatively define at least portions of a particular area within thesame hemisphere of said brain.
 25. The apparatus of claim 11, whereinsaid sensors are adapted to be applied to the outside periphery of thetest subject and to operate non-invasively.
 26. A method for concurrentcomparative in vivo monitoring of blood metabolites in each of aplurality of different internal regions in a selected test subject,comprising the steps of: spectrophotometrically irradiating each of aplurality of different testing sites on said test subject; detectinglight energy resulting from said spectrophotometric irradiation of saidtesting sites, and providing separate sets of signals to a control andprocessing station which are representative of the light energy receivedby each of said testing sites and which cooperatively define bloodmetabolite data for an individual one of at least two different internalregions; analyzing said separate signals to determine quantified bloodmetabolite data representative of at least one defined region withinsaid at least one test subject associated with each of at least twodifferent of said testing sites, each said defined region beingdifferent from the other; and concurrently displaying data sets for eachof said at least two different internal regions at substantially thesame time for direct mutual comparison, wherein said at least twodifferent internal regions are located within different brainhemispheres of said test subject.
 27. The method of claim 26, whereinsaid data sets include a first set which characterizes a first zonewithin one of said at least two different internal regions and a secondset which characterizes a second zone that is at least partially withinthe same one of said at least two different internal regions.
 28. Themethod of claim 26, wherein said spectrophotometric irradiationcomprises application of at least two different wavelengths applied inan alternating sequence of timed pulses, and wherein detection of lightenergy corresponding to each of said at least two different wavelengthsis done on a timed periodic basis using detection periods whoseoccurrence generally corresponds to that of said appliedspectrophotometric irradiation.
 29. The method of claim 28, wherein theduration of each of said detection periods is limited to a length whichis less than that of each pulse of applied spectrophotometricirradiation.
 30. The method of claim 29, wherein the duration of each ofsaid detection periods is less than half that of a pulse of said appliedspectrophotometric irradiation.
 31. The method of claim 30, wherein aplurality of said detection periods are used during pulses of saidapplied spectrophotometric irradiation, and a corresponding energydetection occurs during each of a plurality of said detection periods.32. The method of claim 31, further including the steps of averaging aselected number of energy detection event values to obtain a resultantvalue therefor, and using said resultant value to compute a metabolitevalue which is representative thereof.
 33. The method of claim 32,wherein said display includes said computed representative metabolitevalue.
 34. The method of claim 33, wherein said display is refreshedperiodically by using a sequence of computed representative metabolitevalues which are based upon and represent the averaged detection eventvalues produced during the different time intervals corresponding to theintervals of said periodic display refreshment.
 35. Apparatus forspectrophotometric in vivo monitoring of a selected metabolic conditionin each of a plurality of different test subject regions on asubstantially concurrent basis, comprising: a plurality ofspectrophotometric emitters, each adapted to separatelyspectrophotometrically irradiate a designated region within a testsubject from a test location on said test subject; a controller andcircuitry coupling each of said emitters to said controller forindividually operating selected ones of said emitters tospectrophotometrically irradiate at least two particular regions withinthe test subject; a plurality of detectors, each adapted to separatelyreceive light energy resulting from the spectrophotometric irradiationof said at least two particular regions, and to produce at least oneseparate set of signals for each one of said at least two particularregions; and circuitry acting to convey said at least one separate setof signals to said controller for analytic processing; said controlleradapted to analytically process said at least one separate set ofsignals to determine separate sets of quantified data representative ofa metabolic condition in said at least two particular regions; and avisual display coupled to said controller and adapted to displayseparate representations of said separate sets of quantified data foreach of said at least two particular regions in a mutually-comparativemanner and on a substantially concurrent basis, wherein at least two ofsaid at least two particular regions are located in mutually separateregions of a brain of said test subject.
 36. The apparatus of claim 35,wherein said controller includes a computer programmed to analyze saidsignals to separately determine a blood oxygenation state within each ofsaid at least two particular regions.
 37. The apparatus of claim 36,wherein said computer comprises a processor, data buffers, and a timingsignal generator, said data buffers adapted to store data representativeof said blood oxygenation state and said timing signal generator adaptedto control actuation of said emitters and detectors.
 38. The apparatusof claim 36, wherein said controller comprises a unitary device whichincludes said computer and said display.
 39. The apparatus of claim 38,wherein said unitary device further includes a keyboard interface tosaid computer.
 40. The apparatus of claim 38, wherein said unitarydevice further includes a data output interface.
 41. The apparatus ofclaim 40, wherein said unitary device further includes an integralkeyboard interface to said computer.
 42. The apparatus of claim 38,wherein said display comprises a flat electroluminescent visual displayscreen.
 43. The apparatus of claim 42, wherein said unitary devicefurther includes an integral keyboard interface to said computer. 44.The apparatus of claim 35, wherein at least certain of said detectorsand certain of said emitters comprise operational pairs, and saidcontroller is arranged to operate the emitters and detectors of at leastcertain of said operational pairs in predetermined timed relationshipwhile maintaining the emitters and detectors of other of saidoperational pairs in a non-operating condition.
 45. The apparatus ofclaim 44, wherein said controller is adapted to sequence the operationof said at least certain of said operational pairs.
 46. The apparatus ofclaim 45, wherein at least one of said operational pairs include aplurality of said detectors arranged at mutually spaced locations whichare spaced at differing distances from the emitter of said at least oneof said operational pairs.
 47. The apparatus of claim 46, wherein saidcontroller is adapted to operate the emitter and a selected number lessthan all of the detectors of at least one of said operational pairssubstantially in unison while holding the other detectors of said atleast one of said operational pairs in a non-operating condition, andsaid controller is further arranged to operate said other detectorssubstantially in unison with said emitter at another time during whichsaid selected number of said detectors are maintained in a non-operatingcondition.
 48. The apparatus of claim 44, wherein at least one of saidoperational pairs includes a first detector and a second detector, andwherein the first detector is located nearer the emitter than the seconddetector to thereby provide near and far detector groupings for said atleast one of said operational pairs.
 49. The apparatus of claim 48,wherein said controller is adapted to sequence the operation of said atleast one of said operational pairs.
 50. A regional oximetry systemadapted to substantially simultaneously display tissue oxygen saturationmeasurements for at least two human tissue regions, comprising: a firstsensor including a first emitter, a first near detector, and a first fardetector, the first near detector being located closer to the firstemitter than the first far detector, the first near detector and thefirst far detector each being configured to detect a first lighttransmitted by the first emitter, and the first sensor being configuredto generate a first set of data indicative of the detected first light;a second sensor including a second emitter, a second near detector, anda second far detector, the second near detector being located closer tothe second emitter than the second far detector, the second neardetector and the second far detector each being configured to detect asecond light transmitted by the second emitter, and the second sensorbeing configured to generate a second set of data indicative of thedetected second light; and an oximeter unit including one or moreprocessors, the oximeter unit being configured to: operate the firstemitter of the first sensor and the second emitter of the second sensorin sequence on a substantially simultaneous basis in a manner thatreduces cross-talk between the sensors; receive the first set of dataand the second set of data; determine a first tissue oxygen saturationmeasurement corresponding to a first tissue oxygen saturation parameterbased on the first set of data; determine a second tissue oxygensaturation measurement corresponding to a second tissue oxygensaturation parameter based on the second set of data; and superimpose afirst trace indicative of the first tissue oxygen saturation measurementover a time period and a second trace indicative of the second tissueoxygen measurement over the time period on a display.
 51. The regionaloximetry system of claim 50, wherein the oximeter unit is furtherconfigured to display an event marker in conjunction with thesuperimposed first and second traces.
 52. The regional oximetry systemof claim 51, wherein the oximeter unit is further configured to overlaythe event marker on the superimposed first and second traces.
 53. Theregional oximetry system of claim 51, wherein the oximeter unit isfurther configured to align the event marker with a time indicator. 54.The regional oximetry system of claim 53, wherein the oximeter unit isfurther configured to display the superimposed first and second tracesin substantially real time and to maintain a position of the displayedevent marker with respect to the superimposed first and second tracesand the time indicator over a measurement period of time.
 55. Theregional oximetry system of claim 51, wherein the event marker includesa circle selectively placed on a display.
 56. The regional oximetrysystem of claim 50, wherein the oximeter unit is further configured tooverlay a vertical line on the display to associate an event occurrencewith the superimposed first and second traces.
 57. The regional oximetrysystem of claim 50, wherein the oximeter unit is further configured tosuperimpose the first and second traces on a graph having a verticalscale of oxygen saturation values that range from less than 100% andmore than 0%.
 58. The regional oximetry system of claim 50, wherein thefirst light comprises at least four different wavelengths.
 59. Theregional oximetry system of claim 50, wherein the first emitter of thefirst sensor forms an emitter-detector pair grouping with a third fardetector configured to detect the first light, the third far detectorbeing located farther from the first emitter than the first neardetector, the third far detector being configured to generate dataindicative of the first light detected by the third far detector, andthe oximeter unit being configured to determine the first tissue oxygensaturation measurement using the data indicative of the first lightdetected by the third far detector.
 60. The regional oximetry system ofclaim 50, wherein the one or more processors of the oximeter unit arecoupled to a display generator and to a data output interface.
 61. Amethod for comparatively displaying oxygen saturation measurements of atleast two regions of human tissue, the method comprising: transmitting,with a first emitter, a first light into a first region of tissue of apatient and transmitting, with a second emitter, a second light into asecond region of tissue of the patient, the first light beingtransmitted into the first region of tissue of the patient and thesecond light being transmitted into the second region of tissue of thepatient in sequence on a substantially simultaneous basis; detecting thefirst light with a first near detector and with a first far detector,the first near detector being closer to the first emitter than the firstfar detector; generating a first set of data indicative of the firstlight detected with the first near detector and with the first fardetector; determining, using one or more central processing units, afirst tissue oxygen saturation measurement based on the first set ofdata; detecting the second light with a second near detector and with asecond far detector, the second near detector being closer to the secondemitter than the second far detector; generating a second set of dataindicative of the second light detected with the second near detectorand with the second far detector; determining, using the one or morecentral processing units, a second tissue oxygen saturation measurementbased on the second set of data; and superimposing a first traceindicative of the first tissue oxygen saturation measurement over a timeperiod and a second trace indicative of the second tissue oxygensaturation measurement over the time period on a display.
 62. The methodof claim 61, further comprising a step of detecting the first light witha third far detector that is farther from the first emitter than thefirst near detector, wherein the step of generating the first set ofdata includes generating data indicative of the first light detectedwith the first near detector, the first far detector, and the third fardetector.
 63. The method of claim 61, wherein the first tissue region ofthe patient is a tissue region spaced apart from a brain of the patient.64. The method of claim 63, wherein the first tissue region is a bodyextremity.
 65. The method of claim 61, further comprising a step ofdisplaying an event marker in conjunction with the superimposed firstand second traces.
 66. The method of claim 65, wherein the event markeris a circular event marker.
 67. The method of claim 66, wherein thesuperimposing step is performed in substantially real time, and thedisplaying step includes maintaining a position of the displayed eventmarker with respect to the first trace, the second trace, and a timeindicator.
 68. The method of claim 61, wherein the step of transmittingthe first light into the first region of tissue includes transmitting atleast three different wavelengths of light into the first region oftissue.
 69. The method of claim 61, wherein the step of transmitting thefirst light into the first region of tissue includes transmitting thefirst light through a fiber-optic cable to the first emitter andtransmitting the first light into the first region of tissue using thefirst emitter.
 70. The method of claim 61, wherein the step ofdetermining the first tissue oxygen saturation measurement based on thefirst set of data is performed using a single central processing unit.